Scientists identify the mystery killer behind Ireland's potato famine

Starving people searching for potatoes in a stubble field during the Great Famine (1845-1852) which was caused by the failure of the Irish potato crop and British government inaction.

Scientists have finally figured out exactly what strain of potato blight led to the deaths of more than a million people in Ireland during the Great Famine of the mid-19th century — and it's not the usual suspect.

For decades, researchers assumed that a particular strain of Phytophthora infestans, known as US-1, made the leap from the Americas to mainland Europe, and then to Ireland in the 1840s. Selective breeding and fungicides have made US-1 less of a threat than it was a century and a half ago, but it and other strains of blight continue to pose a threat to potato crops around the world. Blight can still turn seemingly healthy potatoes into black, stinking balls of mush, just as it did in 19th-century Ireland.

An international team of scientists took on the task of tracing the roots of late blight through genetics, and to flesh out the story, they deciphered the genomes for 11 strains of blight preserved in Germany's Bavarian State Collection for Botany and London's Kew Gardens. The dried potato plants containing the blight pathogens were saved in herbaria — that is, collections of preserved plants — by 19th-century scientists who had no idea they could yield that kind of scientific data.

What the researchers found surprised them: The genetic signature of the blight that was extracted from the Irish potato plants did not match up exactly with US-1. Instead, the blight represented a closely related but previously unknown strain that has now been designated HERB-1.

Roots of the blightBy mapping the genetic differences between the 19th-century samples and 15 modern-day strains of blight, the scientists could reconstruct the pathogen's evolution over the centuries. They determined that the blight originated in Mexico's Toluca Valley. The species' genetic diversity increased markedly in the 16th century, around the time that Spanish explorers settled the New World. That era marked the wider spread of potato varieties, and probably hastened the evolution of Phytophthora infestans as well.

Kew Gardens

This potato specimen from the Kew Gardens' herbarium was collected in 1847, during the height of the Irish famine. The legend reads "Botrytis infestans" because it was not known yet that Phytophthora does not belong to the mildew-causing Botrytis fungi.

The similarities between US-1 and HERB-1 suggest that they both made their appearance in the early 19th century, not long before the first major outbreak of the blight in Europe. "Probably they both came out of the United States," said one of the study's authors, Sophien Kamoun, a researcher at the Sainsbury Laboratory in Britain.

HERB-1 spread to Europe first, and soon made its way to Ireland, where potatoes were the staple crop for millions of poor farmers. "The potatoes at the time were very susceptible to blight," Kamoun told NBC News. More than a million people died between 1845 and 1852, and at least that many emigrated to friendlier locales. Even today, Ireland's population level has not returned to the pre-famine high of 8 million.

US-1's rise came in the 20th century, after the introduction of new potato varieties that were resistant to HERB-1. Eventually, US-1 became the dominant blight strain, and HERB-1 faded away. "We think HERB-1 is most likely extinct," Kamoun said.

Delving into DNAThe research illustrates how useful herbaria can be for resolving decades-old questions about centuries-old plants. "The degree of DNA preservation in the herbarium samples really surprised us," Johannes Krause of the University of Tübingen said in a news release about the study. It also illustrates how quickly evolution can produce new strains of pathogens, Kamoun said.

"The molecular clock turned out to be shorter than perhaps we expected," he said.

The study's lead author, Kentaro Yoshida of the Sainsbury Lab, said the study suggests that crop breeding methods play a role in the molecular evolution of pathogens.

"Perhaps this strain became extinct when the first resistant potato varieties were bred at the beginning of the 20th century," Yoshida said. "What is for certain is that these findings will greatly help us to understand the dynamics of emerging pathogens."